ABSTRACT
Secondary forests are increasingly important components of human‐modified landscapes in the tropics. Successional pathways, however, can vary enormously across and within landscapes, with ...divergent regrowth rates, vegetation structure and species composition. While climatic and edaphic conditions drive variations across regions, land‐use history plays a central role in driving alternative successional pathways within human‐modified landscapes. How land use affects succession depends on its intensity, spatial extent, frequency, duration and management practices, and is mediated by a complex combination of mechanisms acting on different ecosystem components and at different spatial and temporal scales. We review the literature aiming to provide a comprehensive understanding of the mechanisms underlying the long‐lasting effects of land use on tropical forest succession and to discuss its implications for forest restoration. We organize it following a framework based on the hierarchical model of succession and ecological filtering theory. This review shows that our knowledge is mostly derived from studies in Neotropical forests regenerating after abandonment of shifting cultivation or pasture systems. Vegetation is the ecological component assessed most often. Little is known regarding how the recovery of belowground processes and microbiota communities is affected by previous land‐use history. In published studies, land‐use history has been mostly characterized by type, without discrimination of intensity, extent, duration or frequency. We compile and discuss the metrics used to describe land‐use history, aiming to facilitate future studies. The literature shows that (i) species availability to succession is affected by transformations in the landscape that affect dispersal, and by management practices and seed predation, which affect the composition and diversity of propagules on site. Once a species successfully reaches an abandoned field, its establishment and performance are dependent on resistance to management practices, tolerance to (modified) soil conditions, herbivory, competition with weeds and invasive species, and facilitation by remnant trees. (ii) Structural and compositional divergences at early stages of succession remain for decades, suggesting that early communities play an important role in governing further ecosystem functioning and processes during succession. Management interventions at early stages could help enhance recovery rates and manipulate successional pathways. (iii) The combination of local and landscape conditions defines the limitations to succession and therefore the potential for natural regeneration to restore ecosystem properties effectively. The knowledge summarized here could enable the identification of conditions in which natural regeneration could efficiently promote forest restoration, and where specific management practices are required to foster succession. Finally, characterization of the landscape context and previous land‐use history is essential to understand the limitations to succession and therefore to define cost‐effective restoration strategies. Advancing knowledge on these two aspects is key for finding generalizable relations that will increase the predictability of succession and the efficiency of forest restoration under different landscape contexts.
The massive forests of central Amazonia are often considered relatively resilient against climatic variation, but this view is challenged by the wildfires invoked by recent droughts. The impact of ...such fires that spread from pervasive sources of ignition may reveal where forests are less likely to persist in a drier future. Here we combine field observations with remotely sensed information for the whole Amazon to show that the annually inundated lowland forests that run through the heart of the system may be trapped relatively easily into a fire-dominated savanna state. This lower forest resilience on floodplains is suggested by patterns of tree cover distribution across the basin, and supported by our field and remote sensing studies showing that floodplain fires have a stronger and longer-lasting impact on forest structure as well as soil fertility. Although floodplains cover only 14% of the Amazon basin, their fires can have substantial cascading effects because forests and peatlands may release large amounts of carbon, and wildfires can spread to adjacent uplands. Floodplains are thus an Achilles’ heel of the Amazon system when it comes to the risk of large-scale climate-driven transitions.
ABSTRACT
We synthesize findings from one of the world's largest and longest‐running experimental investigations, the Biological Dynamics of Forest Fragments Project (BDFFP). Spanning an area of ∼1000 ...km2 in central Amazonia, the BDFFP was initially designed to evaluate the effects of fragment area on rainforest biodiversity and ecological processes. However, over its 38‐year history to date the project has far transcended its original mission, and now focuses more broadly on landscape dynamics, forest regeneration, regional‐ and global‐change phenomena, and their potential interactions and implications for Amazonian forest conservation. The project has yielded a wealth of insights into the ecological and environmental changes in fragmented forests. For instance, many rainforest species are naturally rare and hence are either missing entirely from many fragments or so sparsely represented as to have little chance of long‐term survival. Additionally, edge effects are a prominent driver of fragment dynamics, strongly affecting forest microclimate, tree mortality, carbon storage and a diversity of fauna.
Even within our controlled study area, the landscape has been highly dynamic: for example, the matrix of vegetation surrounding fragments has changed markedly over time, succeeding from large cattle pastures or forest clearcuts to secondary regrowth forest. This, in turn, has influenced the dynamics of plant and animal communities and their trajectories of change over time. In general, fauna and flora have responded differently to fragmentation: the most locally extinction‐prone animal species are those that have both large area requirements and low tolerance of the modified habitats surrounding fragments, whereas the most vulnerable plants are those that respond poorly to edge effects or chronic forest disturbances, and that rely on vulnerable animals for seed dispersal or pollination.
Relative to intact forests, most fragments are hyperdynamic, with unstable or fluctuating populations of species in response to a variety of external vicissitudes. Rare weather events such as droughts, windstorms and floods have had strong impacts on fragments and left lasting legacies of change. Both forest fragments and the intact forests in our study area appear to be influenced by larger‐scale environmental drivers operating at regional or global scales. These drivers are apparently increasing forest productivity and have led to concerted, widespread increases in forest dynamics and plant growth, shifts in tree‐community composition, and increases in liana (woody vine) abundance. Such large‐scale drivers are likely to interact synergistically with habitat fragmentation, exacerbating its effects for some species and ecological phenomena. Hence, the impacts of fragmentation on Amazonian biodiversity and ecosystem processes appear to be a consequence not only of local site features but also of broader changes occurring at landscape, regional and even global scales.
Questions: Secondary succession in the tropics can follow alternative pathways. Land-use history is known to engender alternative successional communities, but the underlying mechanisms driving and ...sustaining divergence remain unclear. In this study we aim to answer the following questions: (1) does previous land use act as a filter for species composition in secondary forests; and (2) what are the relative roles of management practices, soil properties and landscape composition in determining species composition? Location: Central Amazon, Brazil. Methods: We sampled trees, shrubs and palms (≥1cm diameter) in 38 early secondary forests (5 yr after abandonment) located along gradients of land-use intensity in five shifting cultivation landscapes. We measured the diameter and height of each sampled plant, identified it to species or morpho-species level and checked if it was resprouting or not. At each secondary forest we also collected soil samples for chemical and physical analyses and estimated the amount of old-growth forest surrounding it (landscape composition). Results: We found that previous land-use intensity determined species composition. With increasing land-use intensity, management practices of cut-andburn and associated reduction in soil quality filtered out seed-dependent species and favoured strong sprouters and species that can cope with low nutrient availability. Landscape composition had a weak effect on species assemblages. We found specific species assemblages and indicator species associated with different levels of previous land-use intensity. As a consequence of these local filters, species α- and β-diversity decreased and therefore early successional communities became more similar to each other. Conclusion: Species composition of successional forests is strongly determined by different land-use intensities. Dispersal limitation has a limited effect on determining the composition of the dominant species. Filtering effects of management practices and soil quality determine the species dominating the canopy at early stages of succession and narrow down the range of species able to colonize and establish. This study highlights how land use shapes successional communities and suggests that alternative successional pathways are determined at early stages of succession. Therefore, accounting for land-use history is crucial to improve the understanding of tropical secondary succession. We present a list of indicator species for different levels of previous land-use intensity that can be used to support conservation and restoration decisions in the Amazon.
The recovery capacity and the successional pathways of tropical forests after anthropogenic disturbance vary considerably and may depend on prior land‐use type and intensity. It is still unclear if ...forests subjected to high intensity impact, such as periodically burned pastures, are capable of restoring their original functional properties.
This study analysed the functional trait dynamics of the dominant species in successional trajectories following two land uses, pasture or clear‐cut, north of Manaus. Fourteen years of demographic data from the Biological Dynamics of Forest Fragments Project were used to determine the dominant species of the two successional trajectories, for which leaf area, leaf dry mass content, specific leaf area and wood density were collected, whereas seed mass was obtained from literature. Community weighted mean of each trait was weighted by basal area determined annually along succession. Prinicpal components analysis was used to analyse the extension and direction of the functional trajectories of plots.
Forests regenerating from pastures increased in wood density through successional time, but other traits did not change significantly. Succession after clear‐cut exhibited increasing leaf dry mass content and seed mass, and decreasing leaf area over time, but no change in wood density. Functional trajectories of plots after clear‐cut were more extensive and directional than those of pasture‐derived plots.
Synthesis and applications. We demonstrate how central Amazonian secondary forests subjected to different land uses show differences in functional trait trajectories, in ways parallel to previously shown changes in biomass, floristic diversity and forest structure. These results indicate that natural recovery of forest functional traits is affected by prior land‐use history, with implications for management and restoration. Thus, natural recovery of forests on abandoned pastures is much slower than clear‐cuts, even though seed sources from mature forests are very close to these areas, and the former may need intervention to counteract the diverted succession.
Resumo
A capacidade de recuperação e os caminhos sucessionais das florestas tropicais após perturbações antrópicas variam consideravelmente e podem depender do tipo e da intensidade prévia do uso da terra. Ainda não está claro se as florestas sujeitas a impactos de alta intensidade, como pastagens periodicamente queimadas, são capazes de restaurar suas propriedades funcionais originais.
Este estudo analisou a dinâmica de atributos funcionais das espécies dominantes em trajetórias sucessionais após dois usos da terra, pastagem ou corte raso, ao norte de Manaus. Quatorze anos de dados demográficos do Projeto de Dinâmica Biológica de Fragmentos Florestais foram utilizados para determinar as espécies dominantes das duas trajetórias sucessionais, para as quais foram coletadas área foliar, conteúdo de massa seca foliar, área foliar específica e densidade de madeira enquanto que a massa das sementes foi obtida da literatura. A média ponderada da comunidade (CWM) de cada atributo foi ponderada pela área basal determinada anualmente ao longo da sucessão. Uma PCA foi utilizada para analisar a extensão e direção das trajetórias funcionais das parcelas.
Florestas regeneradas de pastagens aumentaram a densidade da madeira através do tempo de sucessão, mas outras características não mudaram significativamente. A sucessão após corte apresentou maior conteúdo de massa seca foliar e massa de sementes, além de diminuir a área foliar ao longo do tempo, mas sem alteração na densidade da madeira. Trajetórias funcionais de parcelas após corte raso foram mais extensas e direcionais do que as de parcelas derivadas de pastagens.
Síntese e aplicações. Este trabalho demonstra como as florestas secundárias da Amazônia central submetidas a diferentes usos da terra apresentam diferenças nas trajetórias de seus atributos funcionais, de forma paralela às mudanças anteriormente mostradas na biomassa, na diversidade florística e na estrutura florestal. Estes resultados indicam que a recuperação natural de atributos funcionais da floresta é afetada pela história anterior de uso da terra, com implicações para o manejo e restauração. Assim, a recuperação natural de florestas em pastos abandonados é muito mais lenta do que cortes rasos, embora as fontes de sementes de florestas maduras sejam muito próximas dessas áreas, e as primeiras precisarão de intervenção para modificar a sucessão florestal desviada.
The responses of plant–animal interactions to forest fragmentation can vary. We hypothesized that large-seeded plant species would be more susceptible to forest fragmentation than small-seeded ...species because large-seeded species rely on a few, extinction prone dispersers. We compared seed dispersal of the large-seeded, mammal dispersed
Duckeodendron cestroides and the small-seeded, avian dispersed
Bocageopsis multiflora. The number, percentage, distance, and distributions of dispersed seeds were all reduced in fragments for
Duckeodendron but not for
Bocageopsis. Other fragmentation research in tropical communities supports this hypothesis through three lines of evidence: (1) Large-seeded plant species are more prone to extinction, (2) Fragmentation restricts or alters the movement of large animal dispersers more than small dispersers, and (3) Large and small-seeded species seem to be differentially linked to primary and secondary forest habitats. Therefore, small-seeded plants may be more resilient to forest fragmentation while large-seeded species may be more susceptible and should be a priority for conservation.
ABSTRACT
Naturally regenerating forests or secondary forests (SFs) are a promising strategy for restoring large expanses of tropical forests at low cost and with high environmental benefits. This ...expectation is supported by the high resilience of tropical forests after natural disturbances, yet this resilience can be severely reduced by human impacts. Assessing the characteristics of SFs and their ecological integrity (EI) is essential to evaluating their role for conservation, restoration, and provisioning of ecosystem services. In this study, we aim to propose a concept and indicators that allow the assessment and classification of the EI of SFs. To this end, we review the literature to assess how EI has been addressed in different ecosystems and which indicators of EI are most commonly used for tropical forests. Building upon this knowledge we propose a modification of the concept of EI to embrace SFs and suggest indicators of EI that can be applied to different successional stages or stand ages. Additionally, we relate these indicators to ecosystem service provision in order to support the practical application of the theory. EI is generally defined as the ability of ecosystems to support and maintain composition, structure and function similar to the reference conditions of an undisturbed ecosystem. This definition does not consider the temporal dynamics of recovering ecosystems, such as SFs. Therefore, we suggest incorporation of an optimal successional trajectory as a reference in addition to the old‐growth forest reference. The optimal successional trajectory represents the maximum EI that can be attained at each successional stage in a given region and enables the evaluation of EI at any given age class. We further suggest a list of indicators, the main ones being: compositional indicators (species diversity/richness and indicator species); structural indicators (basal area, heterogeneity of basal area and canopy cover); function indicators (tree growth and mortality); and landscape proxies (landscape heterogeneity, landscape connectivity). Finally, we discuss how this approach can assist in defining the value of SF patches to provide ecosystem services, restore forests and contribute to ecosystem conservation.
•Agricultural intensification reduces cassava yield and increases labor demand.•Agricultural intensification promotes graminoid infestation, aggravating weed problems.•Intensified swiddens hold lower ...agrobiodiversity and simplified management practices.•Reduced labor productivity and swidden size jeopardize swidden cultivation.
Swidden cultivation is one of the most widespread agricultural systems in the tropics. Due to socio-economic changes, swiddens are either abandoned, substituted for other agricultural systems, or intensified. In the region of the middle Amazon river, Brazil, the high market demand for cassava flour (farinha) combined with land scarcity is inducing agricultural intensification. We define agricultural intensification as an increase in the frequency of swidden–fallow cycles and a decrease in the fallow period. In this study, we evaluate the consequences of agricultural intensification for management practices and swidden productivity in one of the main cassava producing areas of the Brazilian Amazon. We used ethnographic and biophysical surveys to characterize the current management practices and to evaluate the effect of repeated swidden–fallow cycles within a short fallow period regime on swidden size, weed infestation and life-form composition, weeding effort and cassava productivity. Our results show that with repeated swidden–fallow cycles cassava yield decreases, weed cover increases and weed composition changes from a tree-dominated to a graminoid-dominated community. Such changes in the weed community result in increased weeding effort, to which farmers respond by cultivating smaller swiddens. Therefore, the ongoing agricultural intensification leads to lower swidden productivity and household income without ensuing clear benefits for farmers. Limited access to fertilizers, herbicides and technical assistance combined with the market demand for a single product hinders adaptation. Broadening market opportunities and improving technical assistance to farmers could raise the diversification of production and sources of income and guarantee higher resilience to the system.
Amazon Rain Forest Succession MESQUITA, RITA DE CÁSSIA GUIMARÃES; MASSOCA, PAULO EDUARDO DOS SANTOS; JAKOVAC, CATARINA CONTE ...
Bioscience,
09/2015, Letnik:
65, Številka:
9
Journal Article
Recenzirano
Odprti dostop
Land-use practices can dramatically shift the trajectories of rain forest recovery. In a 25-year study, Amazon rain forest regenerated following deforestation as long as seed availability and ...seedling recruitment were not interrupted. In contrast, rain forest converted to cattle pastures via cutting and burning prior to abandonment diverted succession, leading to highly simplified stands dominated by a single genus. Annual fires eliminated seedlings, saplings, coppice, and seeds in the soil, except for several Vismia species. Once established, Vismia regenerated by continual resprouting and resisted the establishment of other rain forest species, especially the normal suite of pioneers. Through time, succession both in abandoned clearcuts and pastures increased in stem density and biomass; however, species accumulation and ecosystem services were limited in pastures when compared with those in abandoned clearcuts. Therefore, prescribed burning to maintain pastureland leaves a legacy that is not readily extinguished, whereas abandoning clearcuts engenders an accelerated rain forest regeneration.
Based on eight years of annual censuses in secondary forests in central Amazonia, we compared successional dynamics in areas presenting alternative states due to different land use histories. Sites ...that had been clearcut without subsequent use are dominated by the pioneer genus Cecropia, but their understory is characterized by a diverse species assemblage. In contrast, areas clearcut and then used for pasture are dominated by the genus Vismia, forming nearly monogeneric stands. We evaluated whether such patterns were the outcome of differences in community compensatory trends, leading to a dynamic system of sequential replacement of species in Cecropia stands, and to a persistent stage of succession in Vismia stands. Floristic turnover in Cecropia stands showed strong and consistent negative frequency dependence. In contrast, Vismia stands exhibited little or no frequency dependence, likely due to local competitive interactions or priority effects. In these stands, species of the genera Vismia and Bellucia remained dominant throughout the monitoring period, whereas species initially of low abundance and frequency remained so. Differences in recruitment were the major driver of these alternative states. As species colonization proceeds, we expect dominance in the Vismia stands to diminish, albeit slowly. Our approach proved to be a useful tool for comparing species turnover in systems presenting alternative states.
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